Ball mill with load dispersion bar



Jan. 13, 1959 N. 1 4 HALL 2,868,463

BALL MILL WITH LOAD DISPERSION BAR Filed May 14, 1957 3 Sheets-Sheet 1 Fig. ,1.

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Jan. 13, 1959 N. L HALL 2,868,463

BALL MILL WITH LOAD DISPERSION BAR Filed May 14, 195'! 3 Sheets-Sheet 2 2d Fig. 4';

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Jan. 13, 1959 N. L. HALL 2,868,463

BALL MILL WITH LOAD DISPERSION BAR Filed May 14, 1957 3 Sheets-SlIeet 3 (kL)-(0z):'ru T E INVENTOR:

A United States Patent O BALL MILL WITH LOAD DISPERSION BAR Newton L. Hall, Salt Lake City, Utah Application May 14, 1957, Serial N0. 659,007 r 2 Claims. (Cl. 241---184) This invention relates to horizontal rotary ball, tube, or tumbling mills used for the comminuting or fine grinding of rock, ore, cement, or like material.

This application is a continuation in part of my copending application, Serial No. 548,355, filed November 22, 1955, since abandoned.

The particular feature of invention as described herein, and partially in my co-pending application, relates to ball or tube mills wherein a load of crushed discrete material to be ground is fed to a rotating horizontal cylinder with a clear grinding compartment of uniform diameter, the interior of the cylinder being clear of all features to obstruct a free fall of the mill load. The ball mill load is intermixed with grinding media composed of metal or pebble balls whereby the grinding and tumbling action of the rotating load develops a grind or pulverization of the material to be ground as it passes through, and over the mill load in a tumbling or cascading action. Various grinding elements may be used to form the grinding media which is intermixed and forms a part of the mill load, such as large pieces of rock, ore, metal or plastic balls or bars. The grinding processes of the rotating mill loads are similar in their attritional or impacting actions, all variations of action being adapted to suit the particular material being ground. The mills can be operated in either a wet, or a dry grind condition.

Standard ball mills in common use which operate with circulating loads of material have axial openings for entry and discharge of load material, and any surcharge of the load above the mill axis is automatically discharged from the mill, accordingly, and for metallurgical processing, ball mills carry loads of less than 50 percent of the volumetric capacity of the mill. For grinding of fine granular products such as cement clinker, loads may be carried above the axial line of the mill. Various percentages of loadings are used to suit the particular material being ground and for metallurgical grinding the most common loading used is approximately at 42 percent of the capacity of the mill.

The mill product in discharge passes through one of various types of gratings such as, a grating in the discharge head of the mill; through a grating at the mill axis; or, through an open and ungrated retarding discharge spout.

Ball mills have diameters up to 13' of, with lengths of less than two diameters. Mills of like construction but of greater length are termed tube mills. Ball and tube mills are grinders, taking their places in the plant circuit following the crushers.

The materials used for feeds to such mills are crushed to less than /2 inch in screen size, more popularly to minus 4 mesh. For metallurgical processing, the desired product reaches 250 mesh in screen size. For grinding cement clinker and other granular products, mill loads may becarried in excess of 50 percent of the mill capacity and produce a product averaging 325 mesh in screen size. The extreme fine grinding for cement is seldom objectionable and generally desireable.

the combination and use of a single, independent, freely,

t structiorl:

2,868,463 Patented Jan. 13, 195.9

Ball charges vary according to the material being ground, the larger and heavier balls creating a greater impact action for crushing and the smaller balls develop by attritional action a finer mesh of grind. For crushing effect the ball charge is developed with metal balls from 2 to 5 inches in diameter, and for fine grinding, the customary ball charge is composed of balls or slugs from 1 to 2 inches in diameter.

The approximate weight of balls in a mill charge is 300 pounds per cubic foot, and the weight of the mill pulp, filling the voids between the balls, averages 10() pounds per cubic foot.

A ball mill charge is a heavy compact body of ore to be crushed intermixed with the grinding media forming the mill load, all rotating in a path, normal to the mill axis, and with a mill interior perirnetric speed, or load perimeter speed, averaging 400 feet of travel per minute.

The ascending portion of the mill load, next to the mill shell, rides to a crest of the load and then descends in a cascade to the toe of the load where it joins the load in ascent as before. The descending cascade rests upon the ascending load and its weight blankets the migrating, material of the load and checks its passage along the load for discharge from the mill. All of the mill load is rotating in a path which is normal to the mill axis, and it is against that rotating force and, weight of super-1 imposed cascade upon the ascending portion of the load, that the migrating fines must pass in discharge from the mill.

The migrant fines are blanketed with a compact load weight and must move in directionabnormal to the mill.

rotation to reach the discharge end of the mill, and one of the objects of this invention is to open and expand the mill load to provide a liberty for the load parts to move from the gyratorial path and which the over-lying compacted load confines.

The distinctive feature of this invention is formed in moving, elongated loa-d dispersion bar of uniform diameter, placed within a standard cylindrical ball mill of uniform shell diameter and of clear interior where it can freely enter, join, and mix with the load being processed, ground, and migrated through the mill, serving primarily to disperse, expand and classify the load sections and its elements. The load dispersing bar is a dispersion bar, and is not designed as a grinding bar, any grinding action derived from and by its use is of subordinate consideration.

In this application the term ball mill, as used herein, applies to a ball, tube, or tumbling mill of like conthe single, elongated, freely moving, and independent bar placed within the load of a ball mill as described herein.

One of the objects of this invention is; to develop a mill action in a ball mill which is of greater load action the mill product.

Another object is; to provide an open channel and. spaces within the mill load whereby the transient load action and avoiding anovertime for and an over-grinding of the fines of the mill product.

A further object is; to combine a load dispersion bar within a ball mill and its load whereby the bar may be dominated for position and action by the mill load when under action.

Another object is; to provide a ball mill with a load The term load dispersion bar applies to dispersion bar of simple design which can receive the mill load action with a minimum of wear upon the bar and with a shape which produces a minimum of reaction uponv the required power for mill operation.

A further objectis; to provid: a ball mill with a load dispersion bar which can be reversed for position within the mill, i. e. end to end, to present an unworn side of the bar-fins facing the falling mill load, as required.

A further object is; to provide a ball mill with a single load dispersion bar for operation Within a ball mill casing of uniform shell diameter and of clear interior to operate independently of any feature within the mill, such as, concentric mill shells, bat-fie plates, or the like.

Further objects are; to develop an increased activity of the load parts, enlarging by expansion the section of the load, .and altering the cascading section and action; increasing the grinding and impact actions of the load; and enlarging the mill capacity without materially altering the required power for mill operation.

Other objectives will become apparent as the invention is disclosed. These objects I accomplish are by means of such structure and relative arrangement of parts as will appear by a perusal of the following specifications and claims:

In the following descriptions reference is made to the accompanying drawings wherein:

Figure l is a longitudinal vertical section of a rotary ball mill, taken on line 11 of Fig. 2, and embodying one form of my invention, showing a load dispersion bar in approximate position for load action, the section being without a mill load.

Figure 2 is a cross section of a rotary ball mill taken on line 22, of Fig. l, and showing a portion of the driving gear with a load dispersion bar of comparative light weight in approximate position within a mill load under action. 1

Figure 3 is a cross section of a rotary ball mill, similar to Fig. 2, with a load dispersion bar of comparatively heavy weight, in approximate position. within the mill when under mill load action.

Figure 4 is a longitudinal vertical section of a rotary ball mill, similar to Fig. l, and taken on line 44 of Fig. 5, and embodying one form of my invention.

Figure 5 is a cross section of a rotary ball mill taken on line 5--5 of Fig. 4, with a load dispersion bar of cruciform cross section in approximate position within a mill load under action. Shown also, in dotted lines, is a load segment B-C-E, with a middle ordinate line D-E, bisecting a load dispersion bar with circumference shown to determine proportions of the bar to the length D 'E, for use as a basis of measurement for computations of bar and load.

Figure 6 is a longitudinal section of a load dispersion bar of circular cross section.

Figure 7 is a cross section of a circular bar taken on line 7-7 of Fig. 6.

Figure 8 is a cross section of a load dispersion bar of cruciform section with vanes in quadrant position, taken on line 8-8 of Fig. 4.

Figure 9 is a cross section of the retarding discharge spout taken on line 9-9 of Fig. 4, the liner 7-a having staggered interior vanes which intermittently retard the discharging product from the mill and permit an open channel within the spout without raising the hydraulic level of the load to the top of the vanes 8b.

Figure 10 is a diagrammatic cross section of the interior of a ball mill with a load filling the section to a greater than half mill capacity, the features shown basing the range of size for measurement of the dispersion bar and related load.

Figure 11 is a diagrammatic cross section of the interior of a ball mill, similar to Fig. 10, with a mill load of less than half capacity.

Similar reference characters refer to similar parts in all'of'the views.

Referring to the drawings for further descriptions:

Figure 1 represents a longitudinal vertical section of a ball mill of uniform shell diameter with an open and clear interior and with a load dispersion bar of circular cross section in approximate position for operation, the load within the mill not being'shown. The ball mill cylindrical casing or shell is note-d by the numeral 1; With hollow trunnion mill heads 2, at the feed head and 2%, at the discharge head. The wear liners within the circular shell are noted as 3, and the lifter lugs as 3a. The feed head liner is noted as 4, and the discharge head liner as 4-11. The mill heads are formed with hollow trunnions 2-11 and 2c, and rotate in the bearings 5 and 5-11, and are supported upon the abutments 6 and 6-a.

The hollow feed trunnion 2b has a screw thread 2d, for advancing feed to the mill interior as delivered by the feed scoop 13.

The hollow trunnion of the discharge head 211 is l'ined'v with a discharge spout 8, which holds a grating 7,,for the control of the mill discharge. The mill casing 1 is operated by the drive gear 9, which is motivated by the pinion 10, being operated in turn from the drive shaft 11, and whichis motivated by any suitable power arrangement.

Figure 2 is a cross section taken on line 22, of Fig. 1, showing a load dispersion bar 16, of comparatively light weight for size and its reaction upon the load 14. in operation, the load 14 ascends to the load crest 14-a, Where it descends in an upper cascade 14-b, to meet the upper zone of impact 14c. In a continuing descent the load falls into a lower cascade 14-d, where it meets the toe of the load 14-f, and forms the lower zone of impact i l-e, and then joins the load in ascension and continues as before. The loadli i is of comparatively heavy weight to the bar 16, the load dominates the bar for action and position, and supports the bar for rotation providing an open channel or-k space 15, beneath the rotating bar.

In comparison to the load and bar action of Fig. 2, the similar section of mill and bar is shown in Fig. 3, wherein the bar 16-0: is of heavy weight compared to the load and dominates the load in action, bearing downwards onthe load to the exclusion of the channel beneath the bar and exerts a reaction downwards, and beneath the bar and the adjacent mill shell. The bar of Fig. 2 does not bear downwards, nor is it supported by the load beneath, and does not bear against the mill shell in a downwards reaction. The bars 16 and 16-a are contrary in their actions. The bar 16 of Fig. 2 acts upwards from the *bar and upon the load, creating an overfall over the bar then descending to form the lower cascade 14-d, and upon this over-fall at 14c, the descending load from the load crest 14-a falls to form the upper cascade at 14-b, to meet the upper Zone of impact 14-c. The bar of heavy weight 16-a dominates the load, bearing downwards, and pulsates the load for bar position, fluctuating the load, and places an undue burden upon the operating motor.

In Figure 4, the section is shown with an open discharge trunnion in the outlet and a load dispersion bar of cruciform section with fins in quadrant position, with the bar being of a length less than the interior length of the shell, and of a sufiicient amount to avoid a casting of load parts between the end of the bar and the head lining of the mill at 18 and 18-a. Rotating mill loads will vary in cross sectional shape according to the influences and features within the mill, and the cross sectional shape of the load at 18 and 18a will form according to the sectional shape of a plain load inaction, whereas, that portion of the load which is within the range of the dispersion bar 16-a will have a different sectional shape, and the differing and adjoining. shapes do not have time within a revolution ofthe mill for adjustment, and the operating characteristics of the disturn in mill action and uniformly distribute its reactions on the load with its balancing effect to moderate all sections of the load along the bar length, securing a well balanced and distributed load throughout the length of the grinding compartment. A load dispersion bards of uniform size, weight, and cross section along its length, avoiding irregularities of shape, and unbalanced weights, securing equalized effects upon the rotatingmill load. Figure 7 shows a bar of circular cross section 16, with a composition filling the interior 16-c.

In Figure 5 a cross section of the mill with load is shown similar to Fig. 2, with a load dispersion bar 16-a, of cruciform cross section, the loadactions and sections having similar reference characters.

For the proportioning of the dispersion bar and its encompassing load for weights and measurement, a segment of the mill section B--CE is shown in Fig. 5, with a rise D--E, extending from a middle point on the chord BC, to a point on the circumference, placed in perpendicular alinement. The line DE represents a maximum depth of the mill load, and also represents the middle ordinate, or rise, of the segment. In dotted lines a dispersion bar with encompassing circumference is shown with diameter F-G, forming a base of measurement of the load in comparison to the bar, as further outlined in Figs. 10, and 11.

Figure 9 is a cross section taken on line 9--9 of Fig.

4, with a liner 7-a, having staggered interior vanes S-b, which intermittently retard the discharging product from the mill, returning the oversize to the mill and permit an open channel within the spout for discharge of fines without raising the hydraulic level to the top of the vanes 8-b.

Figure 10 is a diagrammatic cross section of the inte trier of a ball mill of assumed worn diameter, with a load filling the section to a greater than half capacity.

The interior circumference of the mill, the inner perimeter, is noted by the enclosing circle P, with the top of the load at BC, which is the chord of the segment BC-E, and from the middle point D, on the chord BC, the line DE extends perpendicularly to the point on the circumference E, the line DE representing the greatest depth of the load, the middle ordinate, or, rise of the segment, and upon this line D--E, as a basis for measurements, the diameter of the dispersion bar, and, diameter of cylinder encompassing the bar, are proportioned to the depth of the load DE, varying from a lesser diameter of twenty-five percent, to the greater diameter of sixty-five percent of the rise DE.

For the purpose of obtaining the depth of the loadv DE, the internal diameter taken at the mid-center line of the mill can be measured with a degree of accuracy, and the measurement secured from the middle point D, on the chord of the segment, said chord being the top of the load, to the azimuth of the interior of the mill at Z, and the measurement deducted from the internal diameter of the mill K-L, giving the resultant T-U, which is equal to the rise or depth of the load DE.

The size of the load governs the size of the bar which should be used to properly effect the designed re-action of the load. The internal circumference, or perimeter, of the mill is also the outer perimeter of the load, i. e., that portion of the perimeter of the load which is in contact with the mill shell, and, for mills of practical size, the perimetric speed of the load will average just 2,ses,4es

below, or above, 400 feet of travel per minute. Only a slight variation of this speed of travel is required to change the load in descent from a cascade resting upon the ascending portion of the load into a cataracting fall of load which is free from the ascending, or descending cascading load. 1 i

Measurements which are to be taken for a bar diameter from a load under operative action can be determined with a fair degree of accuracy from hand operated models which have a transparent face placed normal to the mill axis, and through this face the comparative size of the bar and action of the load can be observed for adjustment and trial. Methods using visible model operations can well supplement designed calculations.

Mill load-s vary in weight according to the type of material being ground, a light weight pumice may be ground in one mill and have different characteristics of grind and develop a mill action compared to anadjoining mill grinding a heavy ore, and the load dispersion bar must be designed accordingly. Each application of bar is operating under the same fundamental principles for use of the bar although each application may differ widely in comparison of size measurement, weight and shape with out departing from the spirit of the invention.

The load dominates the bar due to the primary fact of its heavier proportional weight, the limit for comparison being the weight of the cross section of the load contained within the cylinder encompassing the bar. 'For example, for a bar and load of 10 foot length, the encompassing ore, i. e. the mill load, may have a Weight of 2,400 pounds, and the weight of the bar at 600 pounds, showing the weight of the bar to be one fourth of the weight of the load contained within the cylinder encompassing the bar, the encircling load and the bar having the same diameter, F-G, in Figs. 5, 8, l0 and 11.

The proportional weight of the bar may vary from one fifth minimum, or there-abouts, to any percentage measuring less than the proportionate weight of the encompassing load, the comparison being in weights of equal displacement.

The position of the bar Within the load when under action is off from the vertical center line of the mill axis, and the bar weight adds to the constant lift required upon the mill motive power. 'The bar is not designed as a weight grinding bar and the weight of the bar can accordingly be held to a minimum which is at most less then the proportionate weight of the load cylinder encompassing the bar.

The action developed by the rotation of the load dispersion bar opens and distends the load effecting a change in load action, providing an upper and a lower cascade with their zones of impact, and distends the load to provide for rapid migrational movement of ground parts and decreasing the time required for accomplishing the designed product.

The over-retention of ground parts within the mill load accounts for the extreme, and not required, retention of the load within the mill. A grinding of the fines to the size necessary to liberate the mineral portion from the gangue of the ore properly marks the finishing point required for grinding operations. The excess of fines within the mill product is frequently in adverse action to the best mineral recovery. Some metallurgical operations have the best mineral recovery with ores ground to mesh of screen size, others carry the grind to 200 mesh, the amount of grind varying with the nature of the ore being ground.

Metallurgical recoveries of minerals have advanced from a sixty percent recovery to over ninety-five percent of the ore, leaving only a small field for future recoveries to be gained, but a vast field is opened for the type of processes to be used with the decreased cost in time of operation. The greatest cost of ore beneficiation lies in the expense of crushing and grinding of the ore.

Within the field of dry grinding, particularly the grinding'of cement, extremely {fine "grinding is'but seldomob jectionable, and generally desireablefor -improving 'the 'strength of the concrete.

'The increased grindingaction secured from load dispersion and 'distention-bythe use of the load'dispersion bar develops a range of mill action which enlarges the scope of milloperation.

I claim:

1, -A rotary ball mill comprising; a-sin'gle cylindrical casing with a clear grinding compartment of uniform shell diameter, said casing :being rotatable'about'a horizontal axis, "and open at both ends for respectively receiving and discharging material, and adapted to'contain a load of material to be ground; and a singleyindependent, elongated bar adapted to be freely movable within and having a lengthslightly less than the length of the interiorof said compartment,-and a weight less than the weight of the volume of material comprisingsaid load that would be displaced by a cylinder submerged in said load, said cylinder having a diameter and length equal to the diameter and length-of said bar, said elongated bar having a uniform diameter equal in magnitude to a measurement lying between twenty-five percent and sixtyfive percent of the depth of said contained load measured in a static position.

2. A rotary ball mill comprising; a'single cylindrical casing with a. 'cleargrinding compartment of uniform shell diameter, said casing being rotatable about-a 'horiweight less than the "weight of the volume of material comprisingsaid'load'that'would be displaced by a cylinder submergedin said load,sai'd cylinder-having a diameter and 'lengthequal to the diameter'and length of said'bar,

said bar being adapted to be submerged in said load to operate freely and betweenthe ascendent and descendent portions of saidload, todisperse and distend said load portions up-wardlyfrom said bar.

References Citedinthe file of this patent UNITED STATES PATENTS 841,728 'Sly L Jan. 22, 1907 970,020 Demarest Sept/l3, 1910 1,907,080 McMillan May 2, 1933 2,557,528 Andrews June 19, 1951 2,680,568

Weston June '8, 1954 

